runtime-Method Swizzling（方法调换）

通过修改一个已存在类的方法, 来实现方法替换是比较常用的runtime技巧。

使所有的类都具有调换方法的功能：

#import <Foundation/Foundation.h>

@interface NSObject (SwzillingMethod)

+ (void)swzillingOriginSel:(SEL)originSel withSwizzlingSel:(SEL)newSel;

@end

#import "NSObject+SwzillingMethod.h"

#import <objc/runtime.h>

@implementation NSObject (SwzillingMethod)

+ (void)swzillingOriginSel:(SEL)originSel withSwizzlingSel:(SEL)newSel{

Method originM =class_getInstanceMethod([self class], originSel);

Method newM=class_getInstanceMethod([self class], newSel);

method_exchangeImplementations(originM, newM); // 方法调换

}

@end

需求：调换UIViewController类系统的ViewWillAppear方法，自定义ViewWillAppear方法

#import <UIKit/UIKit.h>

@interface UIViewController (ViewWillAppear)

@end

#import "UIViewController+ViewWillAppear.h"

#import "NSObject+SwzillingMethod.h"

@implementation UIViewController (ViewWillAppear)

+ (void)load{

static  dispatch_once_t onceToken;

dispatch_once(&onceToken, ^{

[self  swzillingOriginSel:@selector(viewWillAppear:) withSwizzlingSel:@selector(zn_viewWillAppear:)];

});

}

// 自定义ViewWillAppear方法的实现

- (void)zn_viewWillAppear:(BOOL)animated{

// 实现部分自定义，需要在UIViewController类的ViewWillAppear方法中做什么就写啥

NSString*name =NSStringFromClass([self class]);

// 实现：使得类名为ViewController的类中视图背景色为红色

if([name isEqualToString:@"ViewController"]){

self.view.backgroundColor= [UIColorredColor];  

}

//这个一定要切记切记切记!（与重写ViewWillAppear方法类似）

[self  zn_viewWillAppear:animated];

}

@end